The present invention relates to furnaces in applications where NOx emissions must be minimized. This is particularly important in electric utility power generation applications, which are highly regulated by environmental authorities.
An important example of this technology is pulverized-coal burning furnaces. Disclosed herein is an apparatus for dramatically improving the NOx emission characteristics of these furnaces by the concentration of fuel and the subsequent reduction of air proportions available at various stages of the combustion process. The present invention takes advantage of the comparatively slow diffusion of solid fuel particles relative to the reactant oxidizing gases to simultaneously minimize NOx formation and maximize NOx destruction reactions in all phases of solid fuel combustion by increasing the fuel-rich reactive volume both in the near-burner region and throughout the entire furnace. Thereafter, using the derived process methodology, furnace-particular devices are designed to optimize the introduction of solid fuel and combustion air to the furnace, which affects the NOx emissions. The theory behind such devices and their respective design is the separation of much of the air used in entraining and transporting the solid fuel to the burner through the application of force-related processes in the burner.
Coal is the primary fuel for electric utility boilers. For efficiency, coal requires combustion at 3000° F. or higher. Very extensive coal deposits that contain both sulfur and nitrogen are available in the eastern half of the United States, and the use of this coal for power generation is a major source of SO2 and NOx pollution in the Eastern United States. NOx and SO2 are pollutants that lead to smog and acid rain over wide areas far removed from the combustion source, and it is especially a problem in urban environments.
There are two main sources of NOx. One is primarily formed during the combustion of solid coal. The fuel-bound nitrogen whose concentration is generally in the range of 1.0%-1.5%, by weight in the coal, is the primary source of NOx in coal combustion. Additionally, combustion with air in excess of the amount required for stoichiometric combustion, which is required for all fossil fuels to minimize other pollutants, such as unburned fuel particulate and carbon monoxide, results in the formation of thermal NOx. The thermal NOx concentration rises substantially at temperatures above about 3000° F.
In addition, and most importantly, the most significant source of oxygen is through the primary air, which is used to transport and inject pulverized coal into furnaces. Recognizing, then, that both secondary and primary air flows directly influence the NOx emissions in such furnaces, it is an objective of this invention to provide a device which minimizes the mixing of coal with both air flows by the centrifugal separation of pulverized coal from the primary air as it is injected into the furnace through one or more burners. This is achieved by the design and construction of a cyclonic device, as follows.